6 th Evian Workshop December 2015 LHC Emittance
6 th Evian Workshop December 2015 LHC Emittance Growth until Stable Beams Maria Kuhn – 15 December 2015 Many thanks to V. Kain, G. Baud, E. Bravin, B. Dehning, J. Emery, M. Hostettler, E. Piselli, G. Trad.
Emittance Measurements Run 1 LHC o Situation in 2012: − BSRT not available for second half of 2012. − Wire scanner: beam size strongly dependent on gain and filter of photomultiplier (PM). − Emittance from luminosity not consistent with emittance from wire scanners. Example: e from luminosity Fill 3217 econv, 4 Te. V [mm] Wire scanner 1. 84 ± 0. 06 ATLAS 2. 33 ± 0. 12 CMS 2. 63 ± 0. 14 Wire scanner e dependent on PM gain and filter M. Kuhn - 15 December 2015 2
Beam Size Measurement 2015 LHC o For BSRT upgrade see “Beam Profile Measurements”, G. Trad, 16 December 2015, this workshop. o Optics changes in IR 4: same beam sizes at 6. 5 Te. V as in Run 1 at 4 Te. V. o Wire scanner upgrade: − Replaced broken photomultiplier of beam 2. − Upgraded power supply schematics. − Reduced photomultiplier gain dependency on light intensity. Beam size independent of wire scanner setting, no PM saturation! Emittance from luminosity Fill 4585 econv, 6. 5 Te. V [mm] consistent with BSRT and Wire scan 2. 12 ± 0. 27 wire scanners. Emittance can be measured with accuracy better than 10 %. ATLAS 2. 06 ± 0. 28 CMS 2. 33 ± 0. 32 BSRT 2. 32 ± 0. 33 M. Kuhn - 15 December 2015 3
Example: PM Working Point Study LHC o Scan of beam size versus gain and filter setting @ 450 Ge. V. o Measured beam size minus fitted growth from reference settings: Beam size independent of PM settings. No sign of PM saturation. M. Kuhn - 15 December 2015 4
Emittance Preservation 2015: Overview LHC o Intra beam scattering (IBS) in horizontal planes. − Low e growth due to reduced brightness 2015. o 50 ns beams show very little e blow-up, much smaller than during Run 1. − Average convoluted emittance blow-up: 10 % o Large e blow-up for 25 ns beams possibly due to electron cloud effects and beam instabilities. − Average convoluted emittance blow-up: 25 % o Strong e growth in the vertical planes for many test fills. − Mainly during injection plateau and beginning of ramp. M. Kuhn - 15 December 2015 5
Example: Test Fill 4284 LHC 3 single bunches: Ib = 0. 6 – 1. 1 x 1011 ppb, bunch length = 1. 0 – 1. 25 ns IBS matches e evolution in H. Stronger e growth in V. However, less than usually. No measured optics during the ramp/squeeze included in analysis. Wire scanner measurements versus MADX IBS simulations. B 2 V very noisy measurement at flattop (use average of several values). M. Kuhn - 15 December 2015 6
Example: Test Fill 4284 in Numbers LHC o IBS matches e evolution in H. o Stronger e growth in V. Emittance growth through the cycle of bunch 3 (purple) e 450 Ge. V [mm] B 1 H B 1 V B 2 H B 2 V 1. 90 ± 0. 05 1. 71 ± 0. 04 1. 50 ± 0. 10 1. 58 ± 0. 03 e 6. 5 Te. V [mm] De [%] 2. 08 ± 0. 12 9 ± 7 2. 04 ± 0. 13 19 ± 8 1. 65 ± 0. 12 10 ± 10 1. 95 ± 0. 17 23 ± 11 Desim [%] 8 -2 10 -2 o Vertical emittance growth not consistent with IBS. M. Kuhn - 15 December 2015 7
Vertical Growth at Injection Plateau LHC o Typical e growth: 5 % in 10 minutes in both vertical planes. o Similar e growth times with single bunches and 12 bunch trains. − Growth goes down with energy. − Does not depend on chromaticity/octupole settings (2 ≤ Q’ ≤ 15). − Does not depend on brightness (for tested parameter range). Beam 1 @ 450 Ge. V Wire scanner measurements versus MADX IBS simulations. M. Kuhn - 15 December 2015 8
Vertical Growth Induced by Damper? LHC o Last LHC MD block: tested effect of abort gap cleaning, injection slot cleaning, and transverse damper itself on emittance growth. − Single bunches at 450 Ge. V. Abort gab cleaning: no effect. Injection slot cleaning: blows up close-by bunches in horizontal planes. Transverse damper on/off: no effect on vertical e blow-up. − Damper cannot suppress e growth, could be incoherent (no BBQ activity). BSRT measurements M. Kuhn - 15 December 2015 9
BCMS Beams in the LHC o Tested two 25 ns BCMS fills in the LHC in 2015: low statistics − Fill 4555 with nominal bunch intensity: 1. 1 x 1011 ppb and 589 bunches. • − Fill 4585 with reduced bunch intensity: 0. 9 x 1011 ppb and 12 bunches. • Fill Brightness ~ 0. 62 x 1011 ppb/mm. Brightness ~ 0. 48 x 1011 ppb/mm. e 450 Ge. V [mm] e 6. 5 Te. V, ATLAS [mm] e 6. 5 Te. V, CMS [mm] De. ATLAS [mm] De. CMS [mm] 4555 1. 77 ± 0. 09 2. 43 ± 0. 32 2. 55 ± 0. 35 0. 66 (37 %) 0. 78 (44 %) 4585 1. 89 ± 0. 10 2. 06 ± 0. 28 2. 33 ± 0. 32 0. 17 ( 9 %) 0. 44 (23 %) o Large emittance growth for Fill 4555. − However, many instabilities at injection and start of squeeze. o Growth for Fill 4585 comparable with standard 25 ns fills. − But also similar brightness. M. Kuhn - 15 December 2015 10
Emittance Blow-up vs Brightness 2015 LHC o 25 ns fills: standard and BCMS o Comparison of emittance from luminosity and convoluted emittance measured at injection with wire scanners. − Errors include measured b*, crossing and 10 % lumi uncertainty. Convoluted e: • Collision values from ATLAS luminosity • Injection values from LHC wire scanners Average blow-up ~ 0. 5 mm (25 %) M. Kuhn - 15 December 2015 11
Emittance Blow-up vs Brightness 2015 LHC o 25 ns fills: standard and BCMS o Comparison of emittance from luminosity and convoluted emittance measured at injection with wire scanners. − Errors include measured b*, crossing and 10 % lumi uncertainty. o Only two fills with significantly higher brightness and higher e blow-up. − Not enough statistics to conclude whether BCMS/ high brightness lead to more growth like IBS. Convoluted e: • Collision values from ATLAS luminosity • Injection values from LHC wire scanners M. Kuhn - 15 December 2015 12
Summary & Conclusion LHC o LHC beam profile measurement systems in the LHC in much better shape after LS 1. o Emittance can be measured with accuracy better than 10 %. o IBS well describes e growth measured in the horizontal planes for low intensity fills. o Unknown source of e growth in the vertical planes. o Overall e growth through the cycle for 25 ns beams: − 25 % on average (~ 0. 5 mm). − IBS predicts ~ 5 % e growth (0. 1 mm) for 2015 beam parameters. • Assuming 15 min average injection plateau length. o High brightness BCMS fill showed more overall e blow-up than standard 25 ns. − But only data for only one fill. M. Kuhn - 15 December 2015 13
LHC BACK UP M. Kuhn - 15 December 2015 14
Reminder: 2012 Emittance Blow-up LHC o In 2012 LHC was operated with high brightness beams. − Transverse emittance could not be preserved during the LHC cycle. − ~ 0. 4 – 0. 9 mm normalized emittance growth from LHC injection to start of collisions for 50 ns physics beams. − But emittance measurement precision during LHC Run 1 doubtful. Convoluted e: • Collision values from ATLAS bunch luminosity (similar for CMS values) • Injection values from LHC wire scanners • Average of first 144 bunch batch After Technical Stop 3 (TS 3): very high brightness beams from the LHC injectors lead to even larger emittance growth! M. Kuhn - 15 December 2015 15
Emittance Blow-up vs Brightness 2012 LHC o 50 ns standard fills (only stable bunches) o Comparison of emittance from luminosity and convoluted emittance measured at injection with wire scanners. − Errors include measured b* and crossing angle uncertainty. o Brightness larger than for the 25 ns standard beams in 2015. − Emittance growth brightness dependent. M. Kuhn - 15 December 2015 16
2015 LHC Performance with 50 ns Beams LHC o Emittance in collisions derived from luminosity. o Injection emittance of first batch measured with SPS and LHC wire scanners. 50 ns beams show very little e blow-up through the cycle (~ 10 %), much smaller than during Run 1. M. Kuhn - 15 December 2015 17
Damping/ Decoherence Times LHC o Measured damping and decoherence times during MD 3 − With chromaticity 15 and octupoles 20 A Time/Turns decohrence damping B 1 H B 1 V 59 25 B 2 H 65 21 B 2 V 105 35 98 27 G. Kotzian M. Kuhn - 15 December 2015 18
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